An inductor-capacitor (LC) resonator “tank” is a useful component in many integrated circuits (ICs) and IC applications. For example, integrated oscillators typically use such an LC tank to set the center of the resonant frequency. Similarly, voltage controlled oscillators (VCOs) typically use a fixed LC resonator tank (in one case, in combination with a variable capacitance [varactor] diode) to control oscillator frequency. In integrating an LC tank on the same semiconductor die with other circuit elements, the following design considerations are typically important: (i) losses due to parasitic effects (e.g., series resistance of the inductor coil) should be kept low for a high Q resonator; (ii) the resonator should be isolated from noise due to charge transfer in the semiconductor substrate; and (iii) the associated silicon area should be relatively small.
In one conventional approach (see, e.g., U.S. Pat. No. 5,915,188), an inductor and capacitor pattern are formed in a single layer of metal, and then the inductor and capacitor are covered with a dielectric and a top metal to form an integrated resonator. While such a structure can provide a high Q, drawbacks of this approach may include relatively low substrate shielding and a relatively small capacitance value.
In another conventional approach (see, e.g., U.S. Pat. No. 5,492,856), an inductor metal pattern can be formed over an oxide layer over a p-n junction on the surface of a silicon substrate to form a capacitor in parallel with the inductor. However, drawbacks of this approach may include relatively poor shielding and noise coupling due to the required biasing of the p-n junction.
In another conventional approach (see, e.g., U.S. Patent Pub. No. 2003/0142459), a multilayer capacitor structure can be used with the capacitor isolated from the substrate by a dielectric layer. An odd number of plates can be used so that a virtual ground can be formed with the center plate of the multiplate structure. However, an inductor may typically be separately formed in order to make an LC tank using this approach.
Referring now to FIG. 1, a top view diagram showing a conventional single layer spiral inductor is indicated by the general reference character 100. Inductor pattern 102 can be a substantially spiral pattern and the inductor can be formed in a single layer. Via 104 can be used to couple to another metal layer for connection 108. Connection 106 can be made directly in the same metal layer used to form the inductor. Accordingly, a cross-over metal (e.g., connection 108, formed in a different layer of metallization) may be required to allow connections to both sides of the inductor.
What is needed is an integrated LC resonator tank having relatively high shielding from the semiconductor substrate. Further, it is desirable to reduce or minimize parasitic effects on the inductor to provide a relatively high Q resonator. In addition, it is desirable to form an LC resonator structure using conventional integrated circuit processing techniques, without the use of cross-over metal for the inductor connections.